Advertisement

Advertisement

BIG PHYSICS, BIG QUESTIONS –

Leprosy’s decline caused by rise of TB

By John Pickrell

The decline of leprosy in Europe during the Middle Ages may have been due to the rise of a more virulent killer – tuberculosis.

A new DNA study of two dozen skeletons reveals that leprosy victims frequently had TB too. Because TB is more easily transmitted and kills faster than leprosy, victims were more likely to pass on TB before their death.

Leprosy – a bacterial infection of peripheral nerves and skin – wreaked havoc in Europe and the Middle East from biblical times through to the late medieval period. But the disease mysteriously went into decline during the 14th and 15th centuries, shortly before TB rose up as one of the region’s most significant epidemic diseases.

As early as the 1940s scientists had suggested that TB, caused by Mycobacterium tuberculosis, might cross-immunise sufferers from leprosy, caused by the related Mycobacterium leprae. The idea was born when the BCG vaccine for TB occasionally appeared to be effective against leprosy as well.

Advertisement

But until now, no one had rigorously tested the link between the two diseases, says infectious disease expert Helen Donoghue at University College London, UK.

Old bones

To investigate the relationship between the two diseases, Donoghue and an international team of archaeologists and anthropologists tested 24 human skeletons dating from the 1st to the 16th century and drawn from across Europe and the Middle East. They were looking for signs of co-infection.

All the specimens were known to have been infected with either TB or leprosy because of damage on the bones. The scientists then examined each skeleton for DNA evidence of further infection.

The researchers found scraps of DNA from both the leprosy and TB bacteria in 10 of the 24 skeletons analysed (42%), providing strong evidence that victims had suffered from both illnesses.

“[We] realised that we were looking at a fairly common, previously unrecognised phenomenon of co-infection,” says Donoghue. The finding disproves the idea that TB widely inoculated people against leprosy.

Crowded cities

Instead, Donoghue argues that weakened leprosy sufferers would have easily fallen prey to opportunistic TB, and that the latter disease would have killed them before they had a chance to pass on leprosy – the less-virulent infection.

“Over time this would reduce the number of individuals suffering from leprosy, leading to its overall decline,” she told New Scientist. She points out that, contrary to popular belief, “leprosy is only acquired after long periods of relatively intimate contact.”

TB may have become more prevalent in the late medieval period as people crowded into towns and cities in greater numbers, she adds.

“The idea that TB protects against leprosy is a long-standing paradigm” which this study may now turn on its head, says Bruce Rothschild, a skeletal pathologist at the Northeastern Ohio Universities College of Medicine in Rootstown, US. But he cautions that further tests are required to confirm that the bacterial DNA detected in the bones has been correctly identified.

There were 8.2 million new cases of TB worldwide in 2000, while new cases of leprosy, mostly in south-east Asia, totalled 515,000 in 2003.